This invention relates to apparatus for the retreading of vehicle tires.
At the retreading of vehicle tires, a new tread is applied to a carcass and vulcanized therewith by means of a binder layer.
Due to the recent substantial rise of the raw material price, it has become more interesting to retread vehicle tires, above all such for heavy vehicles, but also for passenger cars.
Conventional methods of retreading tires, however, are very time-consuming and, thus, expensive.
Vehicle tires can be retreaded according to two main methods. At one of them a carcass is provided with a new prevulcanized tread, after the old tread has been removed by grinding. At this method a tie-gum layer is placed between carcass and tread. The tire thus provided with a new tread is vulcanized in an autoclave at overpressure and at a temperature of about 60.degree. C. to 150.degree. C. It takes about two to five hours according to this method to attach the tread by vulcanization to the carcass. The heating medium used is a heated fluid, for example steam. There exist different variants of this schematically described method, for example the so-called bandag system, the so-called Vacuum-Vulc System and the so-called Vulcap system, which are described in U.S. Pat. No. 4,157,930. At the second one of the said two main methods, an unvulcanized new tread is positioned about a carcass, whereafter the tread as well as the connection between tread and carcass are vulcanized.
In the aforesaid U.S. patent a method of shortening the vulcanization time is described. According to this method, microwave energy is used for heating the tie-gum layer whereby said layer is vulcanized both against a prevulcanized tread and the carcass. The tie-gum used according to this method has a higher dissipation factor at the vulcanization temperature than the prevulcanized tread, in order thereby to concentrate the heating to the tie-gum layer.
The microwave energy is supplied by an applicator located outside the tire periphery while the tire is being rotated.
By the method according to said U.S. patent the vulcanization time is reduced substantially. The method, however, shows several disadvantages. One disadvantage is that the prevulcanized tread yet is heated much more than desired. Furthermore, during the vulcanization process the entire carcass is heated. These disadvantages are due to the fact, that at the vulcanization temperature, or about the same, the energy is concentrated on the tie-gum layer, because the tie-gum layer then has a higher dissipation factor than the tread and the carcass. In order to rapidly increase the temperature of the tie-gum layer, the carcass is filled with hot water, thereby preventing too great a heat development in the carcass and tread. By heating the carcass and tread, namely, they are subjected to ageing, and as a result thereof their service life is reduced.
At the method disclosed in said U.S. patent, thus, the tie-gum must be selected especially in view of the material of the carcass and tread. It further is necessary, as mentioned, to heat the carcass, and therewith the tie-gum, prior to the application of microwave energy. This method, thus, subjects both the carcass and tread to undesired heating.